The invention relates to a process for separating substantially pure ammonia and substantially pure carbon dioxide from mixtures containing ammonia, carbon dioxide and, possibly, water.
In some chemical processes mixtures containing ammonia and carbon dioxide, and sometimes also containing water, are obtained as by-products. For instance, in the synthesis of melamine from urea, a gas mixture is obtained which, in addition to melamine, also contains ammonia and carbon dioxide in amounts of at least 1.7 tons per ton of melamine. In order to effectively utilize this ammonia and carbon dioxide after separating it from the melamine, for example as recycle to a urea synthesis process, it is in most cases necessary to raise the gases to a higher pressure. Compression of the mixture requires special measures to prevent the condensation of ammonia and carbon dioxide and the deposition of solid ammonium carbamate thereby formed.
For this reason, the gas mixture is usually absorbed in water or in an aqueous solution, which results in the formation of an ammonium carbamate solution which can be pumped to the urea synthesis reactor, sometimes being concentrated by desorption and repeated absorption at a higher pressure. A disadvantage of this procedure is that the water, recycled to the urea reactor together with the ammonia and carbon dioxide, has an unfavorable effect on the urea synthesis reaction.
It has been proposed to separately remove the ammonia and carbon dioxide from the by-product mixtures, and to separately recycle them in order to avoid the formation and deposition of ammonium carbamate. However, the binary system of ammonia and carbon dioxide forms a maximum boiling azeotrope at a molar ammonia-to-carbon dioxide ratio of about 2:1, and therefore cannot be separated by simple distillation. This phenomenon also occurs in the ternary system of ammonia, carbon dioxide and water, and the term azeotrope as used herein should be understood to include this phenomenon in the ternary system as well. Also as used herein, with respect to such binary or ternary mixtures, the term "rich" with respect to ammonia shall be understood to mean that the mixture contains a greater percentage of ammonia than would an azeotropic mixture under the same conditions. Conversely, the term "lean" with respect to ammonia, as used herein, means that the mixture contains a percentage of ammonia the same as or less than the percentage of ammonia in an azeotropic mixture under the same conditions.
Various methods have been proposed to get around this azeotrope while still avoiding the necessity of recycling water together with the ammonia recycled to a urea synthesis process, all of which entail the separation of the ammonia-carbon dioxide mixtures into their constituents.
Some of these processes are based on selective absorption of either the ammonia or the carbon dioxide in a liquid. The Netherlands Patent Application No. 143,063, for example, describes a process in which ammonia is absorbed in an aqueous solution of an ammonium salt of a strong acid, such as ammonium nitrate, at an elevated pressure. Selective absorption of carbon dioxide by washing a gas mixture with an aqueous alkanolamine solution, such as monoethanolamine is disclosed in German Patent Specification No. 669,314. However, all of these processes have the drawback that the absorbed component must thereafter be removed from the absorbent and purified.
It has further been proposed to separate ammonia and carbon dioxide from mixtures of ammonia, carbon dioxide and water by distilling off most of the ammonia in a first step followed by distilling off the carbon dioxide in a second step carried out at a higher system pressure. The term "system pressure" as used herein means the sum of the partial pressures of ammonia, carbon dioxide and water. Processes of this kind are described in U.S. Pat. No. 3,112,177, and in the British Pat. No. 1,129,939.
U.S. Pat. No. 3,112,177 describes a process in which in a first step carried out at a system pressure of between 1 and 5 atmospheres absolute, carbon dioxide gas is separated from a mixture of ammonia, carbon dioxide and water, which mixture is lean with respect to ammonia. The remaining liquid is then stripped with, for instance, methane at an overall pressure of 1 atmosphere absolute. This results in a lowering of the system pressure and in the escape of ammonia and some carbon dioxide, so that a mixture of methane, ammonia and carbon dioxide with an overall pressure of 1 atmosphere absolute is obtained. In order to remove traces of carbon dioxide contained in the gas mixture, part of the mixture is condensed, which causes the carbon dioxide to be absorbed by the liquid ammonia.
A similar process is described in British Pat. No. 1,129,939. According to this disclosure, a gas mixture consisting of ammonia and carbon dioxide, rich with respect to ammonia, is absorbed in water or an aqueous solution. Ammonia is distilled from the resulting aqueous solution at atmospheric pressure. The remainder of the solution is then subjected to fractional distillation at a pressure of between 5 and 20 atmospherees absolute with heating in order remove the carbon dioxide.
These two processes are based on the principle that changing the pressure of a system of ammonia, carbon dioxide and water makes it possible to separate out ammonia at the lower pressure and carbon dioxide at the higher pressure. The ratio between the system pressure in the ammonia removal and the system pressure in the carbon dioxide removal must, in both processes, be between about 1:5 and 1:20, if the separation is to proceed smoothly.
However, these processes have the drawback that if the mixture to be treated is available at a pressure of more than 1 atmosphere, it first has to be expanded to 1 atmosphere. Gaseous ammonia is then released having a maximum pressure of 1 atmosphere, or lower, in the event a large amount of another gas is present. If this ammonia is to be subjected to further processing, such as in a urea synthesis process, it has to be raised to a higher pressure. The compression energy required for this is quite substantial. Furthermore, the carbon dioxide concentration in the ammonia has to be kept extremely low to prevent formation and deposition of solid ammonium carbamate in the compressor and high pressure lines.
Another possibility is to liquefy the gaseous ammonia by deep cooling and thereafter raise the liquid ammonia to the required pressure by pumping. However, this also requires the consumption of a substantial amount of energy.